Instead, they use virtual 3-D cells much larger than water molecules, which behave according to the same physics thought to control crystal growth.

"This is kind of an intermediate approach," says Gravner.

He and Griffeath created their virtual cells, called cellular automata, to be one cubic micron in size.

At that scale the cells, about the size of a speck of dust, mimic the physics of water vapour and crystalline growth.

They then ran the model many times to see what happened when they tweaked with temperatures and vapour pressures.

Stunning

The result was a wide variety of snow crystals, including the complicated and stunning six-sided star crystals.

Each crystal took about 24 hours to build using a powerful desktop computer, Gravner says.

"Some forms are easier to get than others," he says.

In this way the model seems to reflect the predominant crystals seen in nature, Gravner says.

"I think it's a big advance since nobody was able to do it before," Libbrecht says.

"People have tried to get realistic snowflakes and it just didn't work."

Previous attempts succeeded to a certain point, after which the virtual crystals would go nuts, probably because of errors that built up in the computations and overpowered the simulation, Libbrecht says.

"These guys were able to generate some structures that were very well behaved," he says.

Their success is all the more interesting, Libbrecht says, because the details of the physics Gravner and Griffeath programmed into their model are not quite in line with what he and some other physicists think are going on in snow crystal formation.

So either the physicists have been wrong, Libbrecht says, or there's something about the modelling approach that allows it to work despite the physics.